1. Field of the Invention
The present invention relates generally to PIN photo diode active pixel sensors. In particular, the present invention relates to an elevated PIN diode sensor with a simplified upper electrode contact structure.
2. Description of the Background
Image sensors such as charged coupled devices (CCDs) and active pixel sensors are used in a wide range of applications such as digital cameras, camcorders, and night vision enhancement systems. In these applications, light detected at an array of such image sensors is converted to electrical signals that have amplitudes proportional to the intensity of the light. Thus, the image sensors can convert an optical image into a set of electronic signals. The electronic signals may represent intensities of colors of light received by the image sensors. The electronic signals can be conditioned and sampled to allow image processing.
Integration of the image sensors with signal processing circuitry is becoming more important because integration enables miniaturization and simplification of imaging systems. Integration of image sensors along with analog and digital signal processing circuitry allows electronic imaging systems to be low cost, compact, and require low power consumption.
Historically, image sensors have predominantly been CCDs. CCDs are relatively small and can provide a high-fill factor. However, CCDs are very difficult to integrate with digital and analog circuitry. Furthermore, CCD systems dissipate large amounts of power and suffer from image smearing problems.
Active pixel sensors are an alternative to CCD sensors. Active pixel sensors can be fabricated using standard CMOS processes. Therefore, active pixel sensors can easily be integrated with digital and analog signal processing circuitry. Further, CMOS circuits dissipate small amounts of power.
FIG. 1 shows a cross-section of a prior art array of image sensors. This array of image sensors includes PIN diode sensors located over a substrate 10. An interconnection structure 12 electrically connects an N-layer (N-type layer) 14 of the PIN diodes to the substrate 10, such as a silicon substrate. An I-layer (intrinsic layer) 16 is formed over the N-layer 14. A P-layer (P-type layer) 18 is formed over the I-layer 16. The P-layer 18, the I-layer 16 and the N-layer 14 form the array of PIN diode sensors. A first conductive via 20 electrically connects a first diode sensor to the substrate 10, and a second conductive via 22 electrically connects a second diode sensor to the substrate 10. A transparent conductive layer 24 is located over the array of diode sensors. A conductive lead 26 is connected to the transparent conductive layer 24. The conductive lead 26 is connected to a bias voltage that allows biasing of the P-layer 18 of the array of PIN diode sensors to a selected voltage potential.
A limitation of the image sensor structure of FIG. 1 is the electrical connection between the conductive lead 26 and the transparent conductive layer 24. The transparent conductive layer 24 must be electrically conductive to allow biasing of the PIN diodes, and must be transparent to allow the PIN diodes to receive light. Generally, it is very difficult to bond to the types of materials that must be used to form the transparent conductive layer 24. Therefore, the conductive lead 26 must be attached to the transparent conductive layer 24 with the aid of some type of clamp or support structure. The result is an electrical connection which is not reliable and which is expensive to produce.
It is desirable to have an active pixel sensor formed adjacent to a substrate in which a transparent conductor is reliably electrically connected to a pixel sensor bias voltage which originates on the substrate.